I just installed SDDM v3.1 and tried to open a dmd file from another user (I have used it to create my own model, saved it and can open it later). The log says "Open Design: 'filename'", then "Open Design: OK". But I don't see anything in the Logical (Logical Model) window. If I try to Import the same file, I get an error message that says look at the log file for details. Except that the log file has no entries in it. Is there an issue with what version the other file was saved under or some other format? Thanks for any help on this.

Provided the model includes some Entities, they should show up in the Logical Model window.

The log file is normally in file datamodeler.log in folder datamodeler\datamodeler\log (unless some other location has been set in the Environment/Log page of the Preferences (accessed from the Tools menu).

The log should at least contain a line like
2012-10-19 11:03:18,457 [main] INFO ApplicationView - Oracle SQL Developer Data Modeler 3.1.3
for each time you start a Data Modeler session.

It is possible that the log is in a read-only location, and so Data Modeler cannot write to it.
In this case I would recommend that you update the Preferences to use a writable location.